EDM is utilised with regard to processing of workpieces by spark erosion. The workpiece and the electrode (usually made from graphite, copper or brass) are generally presented with a dielectric fluid between them and are connected to a DC power supply (EDM generator) delivering periodic pulses of electric energy, such that sparks erode the workpiece by melting and vaporisation and so create a cavity or hole or otherwise shape a workpiece. In order to provide for spark erosion, the workpiece and the electrode must have no physical contact and a gap is maintained typically through appropriate sensors and servo motor control. Erosion debris must be removed from the erosion site and this usually necessitates a retraction cycle during conventional electrical discharge machining. It is possible to utilise multiple electrodes in a single tool holder to allow several erosion and machining processes to be performed at the same time and normally side by side.
In HSEDM a high pressure (e.g. 70 to 100 bar) dielectric fluid pump is utilised in order to supply dielectric fluid to the gap between the workpiece and the electrode. As a result of the high pressure presentation of the dielectric fluid, the process is more efficient than conventional EDM, allowing more rapid removal of debris such that erosion rates are far greater. With HSEDM there is no need for retraction cycles between stages of erosion for evacuation of debris, as the high pressure flow of dielectric fluid in the gap between the workpiece and the electrode is more efficient for the removal of debris produced by the erosion process. Thus generally the electrode is simply fed forwards at a speed necessary to achieve the desired rate of material erosion and removal in accordance with the machining process. Continuous operation results in a significantly-faster machining process.
In the attached drawings, FIG. 1 schematically illustrates a typical HSEDM arrangement for the drilling of holes. The arrangement 1 comprises an electrode holder 2 which presents an elongate electrode 3 to a workpiece 4. Electrical discharge from the tip of the electrode is provided through a direct current electrical power generator 5 such that a cavity or hole is drilled, formed or machined into the workpiece. Dielectric fluid is supplied at a relatively high pressure (70 to 100 bar) to the cavity or hole defined progressively by a spark gap between the electrode and the workpiece. This high pressure dielectric flow is achieved through a pump 6 which acts on a dielectric fluid supply 7 to force the fluid under pressure as indicated into the gap between the electrode and the workpiece. The high pressure flushes and removes debris caused by the discharge process. A servo motor 8 or other device forces continuous movement of the electrode in the length direction of the electrode, driving the electrode into the workpiece. By monitoring the gap voltage, the servo motor can maintain a gap of constant size. Due to the high pressure dielectric fluid flow, there is rapid removal of debris and therefore generally it is not necessary to have a retraction cycle of the electrode in order to allow flushing as with conventional EDM. Thus, in the normal course of events, the servo motor simply moves the electrode down at the speed necessary to keep up with a desired rate of material removal and/or erosion. The constant motion produced by the servo motor allows for rapid drilling, but if drilling is too rapid there is an increased likelihood of short circuiting. In such circumstances, the servo motor retracts the electrode to allow clearing of the electrical short circuits and debris, and then reintroduces the electrode to reestablish the correct gap size for erosion.
HSEDM is used for drilling cooling holes and other features in turbine blades for gas turbine engines. Components such as turbine blades have very strict requirements with regard to hole geometry and surface integrity which can be met by HSEDM. However, HSEDM has high production costs and can lead to large variations in typical breakthrough time to form a hole. Also electrode wear necessitating re-working of components can be a problem. For example, it is not uncommon to have relative electrode wear factors which are greater than 100%, i.e. a greater length of electrode can be worn away than the depth of drilled hole. Electrode wear can also lead to tapering of the electrode, as illustrated in FIG. 2(a), and uneven wear in banks of electrodes, as illustrated in FIG. 2(b). Electrodes that become tapered produce tapered holes, with a restriction at an exit end. Uneven electrodes in a multiple electrode tool result in some electrodes not fully penetrating the workpiece to leave blocked holes. Alternatively, if the servo motor needs to feed the electrodes deeper to complete the hole formation, the excess electrode length in some of the electrodes can lead to backwall impingement erosion and so damage other parts of the component. Such backwall impingement erosion is illustrated in FIG. 3, in which the drilled through-hole 21 in turbine blade 22 continues in the drilling direction 20 into a backwall as unplanned cavity 23. Thus skilled operation of the HSEDM process can be essential.
WO 2009/071865 proposes an improved HSDEM process in which ultrasonic cavitation is induced within the pressurised dielectric fluid flow to enhance debris removal and thereby improve continuous machining.